Commercial production of azelaic acid and pelargonic acid has been realized via an oxidative cleavage of an alkenyl (—C═C—) unit in oleic acid. For example, azelaic acid has been prepared from oleic acid by oxidation with chromium sulfate, as disclosed in U.S. Pat. No. 2,450,858. However, because stoichiometric use of chromium reagents is undesirable, a more efficient approach utilizing ozone has been developed, as disclosed and described in U.S. Pat. Nos. 2,813,113; 5,801,275; 5,883,269; and 5,973,173.
The basic process will be best understood by referring to the description in the accompanying FIG. 1, which is a diagrammatic flow chart indicating the pieces of equipment used and their relationship in the ozonolysis process. Referring to FIG. 1, oleic acid is supplied to a feed tank 10 and then to an ozone absorber 13, wherein the oleic acid is flowed counter-current to a continuous flow of a gaseous mixture comprising oxygen/ozone gas introduced to the ozone absorber 13. The ozone absorber 13 is cooled or refrigerated to substantially control the temperature of the reaction occurring therein.
The ozone absorber 13 receives ozonized oxygen gas by a continuous closed system through which the oxygen circulates. Thus, a given quantity of oxygen is used and reused multiple times and the system need be bled and fed with makeup oxygen only to a small extent to maintain the oxygen content at a predetermined high level by replacing oxygen consumed after a portion has been converted to ozone. The circulating oxygen system comprises an oxygen supply 16 that leads to a dehydrator 19. From the dehydrator 19, the oxygen is transferred to an ozone generator 22, which converts a quantity of the oxygen to ozone by using electricity. From the ozone generator 22, a gaseous mixture containing ozone and oxygen passes into the ozone absorber 13 in which substantially all of its ozone content is absorbed by the oleic acid as further explained below. During the residence time of the oleic acid ozonide-containing mixture in the ozone absorber 13, the mixture may increase in viscosity. If desired, the viscosity of the mixture may be reduced by introducing compatible diluents, such as pelargonic acid, as discussed further below.
Upon exiting the ozone absorber 13, the gas mixture, now substantially devoid of ozone, passes to an electrostatic precipitator 25, which removes any fine mist organic matter that may have been picked up in the ozone absorber 13. The purified gas mixture then passes from the electrostatic precipitator 25 through a compression pump 28 to a cooler 31 and then returns to the dehydrator 19, in which substantially all moisture is removed from the gas mixture. Between the cooler 31 and the dehydrator 19, oxygen-containing gas, which may be controlled by an ozone generating system valve 34, may be supplied to the ozonide decomposing system reactor 37.
The aforementioned absorption of ozone by oleic acid forms oleic acid ozonides, which are transferred to the ozonide decomposing system reactor 37 and treated with oxygen bled from the ozone generating system valve 34. The ozonide decomposing system reactor 37 may be any type device which is adapted to provide substantial interfacial contact between a liquid and a gas and which may be cooled to moderate the temperature of the reaction. The oxygen bled from the ozone generating system is fed into the bottom of the ozonide decomposing system reactor 37 and is agitated with the liquid in each tank by means of mechanical agitators which are not shown.
While only one integral ozonide decomposing system reactor 37 is shown in the drawing, it is to be understood that the reactor 37 may comprise distinct regions configured for independent temperature control, independent pressure control, or both. Alternatively, any number of reactors may be used depending upon the size of the reactors, the rate of the flow of the ozonides and their decomposition products, and the efficiency of the agitation in effecting contact between the oxygen gas and the liquid being treated. Further, alternative embodiments having more than one reactor may be connected in a series configuration, a parallel configuration, or both.
Temperature control is an important operating parameter for the ozonide decomposing system reactor 37. More specifically, the incoming stream of ozonides must be heated to reach a suitable reaction temperature at which the ozonide moiety may efficiently undergo oxidative decomposition upon exposure to one or more catalysts to preferentially form an aldehyde and a carboxylic acid. The ozonide decomposition catalysts may include Brønsted-Lowry acids, Brønsted-Lowry bases, Lewis acids, Lewis bases, metals, or salts and soaps thereof. Exemplary ozonide decomposition catalysts may include at least in part, Na, K, B, Sn, Zn, Pt, Pd, Rh, Ag, Mn, Cu, Ni, titania/silica or titania/P2O5 composites, and combinations thereof. The catalyst can be introduced into the process in the form of a soluble material or in the form of a solid or supported catalyst.
After reaching a suitable reaction temperature, further oxidation of the aldehyde functional group to an acid functional group may occur at a rate sufficient to generate heat, which may in turn contribute to elevating the temperature of the incoming stream of ozonides. However, cooling water may need to be supplied in order to prevent the temperature from rising above a predetermined level. As such, the temperature is controlled in order to be suitable for efficient oxidation to convert the ozonides to mixed oxidation products. In FIG. 1, the heating and cooling apparatus are not shown.
From the ozonide decomposing system reactor 37, the mixed oxidation products pass to a first distillation unit 40 wherein pelargonic acid and other carboxylic acids are distilled from the mixed oxidation products to form a first distillate and a first residue of the mixed oxidation products. The first distillate, which contains pelargonic acid, is converted to a liquid in a first condenser 43 and then is delivered to a crude pelargonic acid storage tank 46. However, some of the crude pelargonic acid may be used to dilute the oleic acid reactant and the oleic acid ozonides in the absorber 13 if desired. Thus, pelargonic acid, which may be crude or further purified, may be added to the ozone absorber 13 in order to reduce the viscosity of the ozonides in the absorber 13. The amount of recycled pelargonic acid supplied to the absorber 13 may be controlled with a valve 49.
It should be noted that other viscosity reducers and diluents may be used. The diluents can be known materials which do not readily react with ozone and which are compatible with the ozonides or the reaction products, or can be a portion of the reaction product. Such diluents include, but are not limited to, saturated short chain acids such as acetic acid, butanoic acid, caproic acid, heptanoic acid, caprylic acid, pelargonic acid, and capric acid; esters such as ethyl acetate and butyl acetate; and alkanes such as hexane, octane, and decane. However, the use of pelargonic acid is recommended because, as an end product of the process, it does not interfere with the operation of the circulating oxygen system and requires no separate distillation. In other words, since pelargonic acid is one of the end products of the process, it is an ideal diluent.
The first residue of the mixed oxidation products, now stripped of a substantial portion of the available pelargonic acid, is next conveyed to an azelaic acid distillation unit 52 in which a portion of the first residue of the mixed oxidation products is distilled to form a second distillate, which includes azelaic acid, and a second residue of the mixed oxidation products. The second distillate is condensed by passage through an azelaic acid distillate condenser 55 to form a crude azelaic acid, which is transferred to a crude azelaic acid storage tank 58. The second residue of the mixed oxidation products or pitch that remains after distilling away the second distillate is removed from the azelaic acid distillation unit 52 and transferred to residue storage 61. The second residue of the mixed oxidation products may still contain some amount of azelaic acid, so further processing, if desired, can occur to recover a portion thereof.
The crude azelaic acid condensate may also contain a wide variety of by-product acids (BPA), such as monocarboxylic acids of undetermined identity with the majority being C6 to C18 monocarboxylic acids. These monocarboxylic acids usually comprise 15 to 20% of the crude azelaic acid condensate. The next step in the process is to purify the crude azelaic acid.
From the crude azelaic acid storage tank 58, the crude azelaic acid is transferred to extractor 64 where the crude azelaic acid is extracted with hot water (e.g., about 175° F., about 80° C. to about 210° F., about 99° C.) to form a hot aqueous solution of azelaic acid. The by-product acids (BPA) that do not dissolve in the hot aqueous azelaic acid solution are decanted from the extractor 64 to BPA storage 67. Meanwhile, the hot aqueous azelaic acid solution is transferred to an evaporator 70 in which water is removed therefrom. Next, azelaic acid in molten form is fed from the evaporator 70 to a flaker 73 where the temperature is reduced to below the melting point, and then solid flakes of azelaic acid are conveyed to an azelaic acid storage bin 76.
While the process and apparatus described above provide azelaic and pelargonic acids from oleic acid, deficiencies exist with respect to the final purity and yield of the azelaic acid. For example, the method described above yields a final azelaic acid with an undesirable level of monocarboxylic acid impurities, which will interfere with the linear propagation for certain polymerization processes by terminating the chain growth. While various crystallization and/or distillation techniques have been utilized to attain the desired levels of monocarboxylic acid impurities, they result in lower product yield and/or lower quality product due to undesirable coloring, respectively. As such, new and/or improved processes and apparatus are needed.